1. Field of the Invention
This invention relates generally to underwater erosion control systems, and more particularly, to a frame for supporting and anchoring an underwater erosion control system and to a method for installing the system.
2. Description of the Related Art
Erosion prevention and control systems are useful for minimizing erosion around underwater structures, including pipes, pilings, bridges and cables, that rely on the seabed for support and also for minimizing coastal shoreline and beach erosion. Methods and devices for preventing underwater bed and shoreline erosion are known. Some of these devices, such as breakwaters and groynes, are effective in minimizing shoreline erosion and are generally constructed from rock, concrete, rubble mounds and other hard body materials. Other devices, primarily used for erosion control on seabed structures, operate by increasing viscous drag on the underwater current, thereby reducing the velocity of the current and of the particulate transported by the current. This causes some of the particulate to settle out of the current and to be deposited in or around the erosion control system. The precipitated particles form a berm in and around the erosion control system. Typical of those devices that increase viscous drag on the current are buoyant frond elements or artificial seaweed or some other viscous drag element. The viscous drag elements are generally secured to the seabed or riverbed via some type of anchor line.
An example of a viscous drag type of underwater erosion control system using anchor lines is shown in U.S. Pat. No. 5,176,469 to Alsop. This system comprises a continuous sequence of buoyant fronds arranged side by side to form a frond line. The frond line is folded back and forth to form an array of fronds, and the successive folded sections of the frond line have aligned openings threaded by anchor lines. The anchor lines secure the array to the sea floor.
A problem with the above system is that the anchor lines do not provide a sufficient amount of structure to maintain the fronds or viscous drag elements in a desired relationship to one another. Additionally, the water currents cause a substantial amount of stress that may tear the openings through which the anchor lines are threaded. Further, anchor lines alone are insufficient to hold the fronds or viscous drag elements in contact with the sea floor, causing adverse effects, including sediment loss due to scour produced by the gap between the sea floor and the viscous drag elements. This problem becomes more severe as the buoyancy of the material used for the fronds or viscous drag elements is increased.
As the relative buoyancy of materials used to construct viscous drag elements increases, the amount of disruption to laminar flow also increases, as does the ability of the material to remain vertical when installed in the water. These factors lead to the enhanced ability to build higher berms of precipitated particles. For coastal shoreline applications it is essential to have high berms which will form submerged, wide-crested breakwaters which, optimally, reach a height of 80% of the depth of the water.
In existing systems, highly buoyant drag elements do not remain in contact with, or in close proximity to, the sea floor preventing the formation of a berm. Thus, these systems cannot be used in coastal shoreline erosion prevention applications.
Therefore, a need exists for a frame and anchoring structure that secures and maintains the viscous drag elements, e.g., panels of highly buoyant material, fronds, or artificial seaweed, in a desired relationship to each other and the sea floor and more uniformly distributes forces from the water current throughout the underwater erosion control system. Where highly buoyant viscous drag elements are required, a need exists for a frame that maintains the erosion control system substantially in contact with the sea floor to allow the formation of a berm of significant height for use as a submerged breakwater in coastal shoreline erosion control applications.